Why do I need a frequency in the offset cell even for a simplex channel? I was redoing my channel list yesterday

and noticed the radio would not transmit on a channel that was for simplex crossband use into my base radio. I had removed the frequency figuring that if it is simplex it doesn't need an offset entered since it is the same as the RX/TX entries. Well that's not

the case. It's no biggie. But I would like to understand the logic of needed the simplex frequency entered into the offset cell. Or maybe I'm doing something else wrong?